Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. In particular, the ease with which films produce fibers (i.e., fibrillate) can be correlated to the degree of molecular orientation of the polymer fibrils that make up the film.
Porous and microporous films have been prepared by extruding thermoplastic polymers, followed by orientation of the film. U.S. Pat. No. 5,811,493 (Kent) describes extruding a composition comprising a polyester continuous phase, a thermoplastic polyolefin discrete phase and a polyester-polyether diblock compatibilizer, followed by orientation. U.S. Pat. No. 4,921,652 (Tsuji et al.) describes melt-molding a blend of an inorganic fine powder with a polyolefin, followed by stretching. U.S. Pat. No. 4,377,616 (Ashcroft et al.) describes a biaxially oriented polymer film comprising a voided, thermoplastic matrix material where the voids are created within the matrix by inclusion of spherical void-initiating solid particles which are incompatible with the matrix material.
Orientation of semicrystalline polymeric films and fibers has been accomplished in numerous ways, including melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.